Segregation of Ca ions at the MgO(001) surface studied by neutral beam incidence ionscattering spectroscopyR. Souda, T. Aizawa, Y. Ishizawa, and C. Oshima Citation: Journal of Vacuum Science & Technology A 8, 3218 (1990); doi: 10.1116/1.576566 View online: http://dx.doi.org/10.1116/1.576566 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/8/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in A soft xray standing wave measurement system for analyzing compound semiconductor surfaces prepared bymolecular beam epitaxy Rev. Sci. Instrum. 67, 3182 (1996); 10.1063/1.1147442 A lowenergy electron diffraction investigation of the surface deformation induced by misfit dislocations in thinMgO films grown on Fe(001) J. Appl. Phys. 80, 2650 (1996); 10.1063/1.363181 Model surface studies of metal oxides: Adsorption of water and methanol on ultrathin MgO films on Mo(100) J. Chem. Phys. 96, 3892 (1992); 10.1063/1.461893 Cation termination at ionpolished and chemically etched (001)YBa2Cu3O7 crystal surfaces: An ion channelingstudy Appl. Phys. Lett. 58, 777 (1991); 10.1063/1.104515 Surface transient behavior of the 3 0Si+ yield with angle of incidence and energy of an O+ 2 primary beam J. Vac. Sci. Technol. A 3, 1359 (1985); 10.1116/1.572778

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Segregation of Ca ions at the MgO(001) surface studied by neutral beam incidence ion scattering spectroscopy

R. Souda, T. Aizawa, andY. Ishizawa National Institute/or Research in Inorganic Materials. 1-1 Namiki. Tsukuba. Ibaraki 305. Japan

C. Oshima School of Science and Engineering. Waseda University Nishiwaseda 1-6-1. Shinjyuku-ku. Tokyo 160. Japan

(Received 18 December 1989; accepted 3 March 1990)

Low energy ion scattering with the use of a neutral Heo beam (NBISS) has been applied to analyze the structure ofMgO(OOI) with Ca2 + impurities segregated from the bulk. The NBISS technique successfully avoids specimen charging, which is a common obstacle to analyzing the insulating surface by using ordinary ion scattering techniques. It is found that the segregated Ca2 + ions are substituted for the Mg2 + ions at the surface and are protruding from the original MgO plane by 0.4 ± 0.1 A.

I. INTRODUCTION

Impurity segregation to interfaces is a well established phe­nomenon that can affect material properties, e.g., strength, conductivity, heterogeneous catalysis, and corrosion. A knowledge of impurity segregation, therefore, seems to be an essential prerequisite for understanding and eventual con­trol of these important processes. Most studies of segrega­tion have been concerned with metal surfaces and alloys I but other materials like simple oxides have been investigated in the last decade. 2

.3 Among them, MgO is thought to be of

essential importance since it has been the subject of many studies, such as catalysis and sintering.4

•5 Segregation of

Ca2 + ions to the MgO(OOI) surface has been studied by low energy ion scattering spectroscopy (ISS) and Auger elec­tron spectroscopy (AES) 6; the surface composition and the effective enthalpy of equilibrium segregation of Ca2

+ ions have been deduced from the temperature dependence of the detailed measurements.

The atomic structure of ionic surfaces has been rather lit­tle studied compared with the extensive work on metal and semiconductors. In terms of MgO(OOI ), as-cleaved and an­nealed surfaces have been studied by low energy electron diffraction (LEED) and reflection high energy electron dif­fraction (RHEED) and the rumpled structure has been in­vestigated. 7-

9 Impact collision ion scattering spectroscopy (lCISS) has recently been applied to investigate the atomic structure of this surface. 1O Schematically shown in Fig. 1 is the atomic arrangement of the MgO (00 1) surface contain­ing Ca2 +- ions. Since the ionic radius ofthe Ca2 + ion is about 1.5 times as large as that of the Mg2 + ion, the substitution of Ca2 + for Mg2 t- necessarily causes lattice strain, especially when Ca2 + is at a bulk site. Indeed, the segregation ofCa2

+

to the surface may occur to buffer the lattice deformation. In this paper, we demonstrate the structure analysis of the Ca2 + segregated MgO(OOI) surface by using NBISS.II Ion scattering performed by conventional ion bombardment techniques may produce spurious results from insulating surfaces like MgO because of specimen-charging effects, and hence to compensate the positive charge, the measurements have been done during electron bombardment lO or by heat-

ing the specimen.6 With the use of an energetic neutral beam, on the other hand, the charging effect of the insulating surface can be minimized. I I Since a part of the incident neu­trals can be ionized during the collision with target atoms,II-13 the NBISS experiment is performed by detecting the ions scattered from the surface.

II. EXPERIMENT

The details of the experimental setup have been described elsewhere13 and only the features important to this experi­ment are described here. The NBISS experiments were made inan ultrahigh vacuum (UHV) chamber (3x 10 II Torr in

MgO (001)

FIG. I. Atomic arrangement of the (001) surface ofMgO including Ca'-' impurities; the circles are drawn proportional to the ionic radius of each ion.

3218 J. Vac. Sci. Techno!. A 8 (4). Jul/Aug 1990 0734-2101/90/043218-06$01.00 © 1990 American Vacuum Society 3218

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3219 Souda etal.: Segregation of Ca Ions at the MgO(OO1) surface 3219

base pressure) equipped with a Heo neutral beam source. The Heo beam was generated by passing an energetic He +

beam, produced in a discharge-type ion source, through He gas introduced into a charge-exchange cell placed between the ion source and the sample chamber; residual He + was removed by an electrostatic deflector at the entrance of the sample chamber. The Heo beam was incident upon a surface with a certain glancing angle a, and He + ions scattered at o L with respect to the primary beam direction were detected by using a rotatable hemispherical electrostatic analyzer.

The material used in this investigation was a single crystal of MgO including 210 ppm Ca impurities by weight. The (001) surface about 10 mm2 was cleaved in the laboratory, and immediately introduced into the ultrahigh vacuum

'I-

o

>-

11\ C ell -C

MgO (001) - HeO

(a)800·C

300 400 500 600 700 E(.V)

FIG. 2. Energy spectra of He + scattered from the MgO(OOI) surface an­nealed at different temperatures. The NBISS experiments were made by using an incident Heo beam with an energy of I ke V. The inset schematically shows the experimental geometry.

J. Vac. Sci. Technol. A, Vol. 8, No.4, Jul/Aug 1990

(UHV) system. The sample could be heated by electron bombardment of the rear face on which platinum was pre­viously evaporated. The clean MgO(OOI) surface was ob­tained by repeated flash heating up to 800 DC and showed an excellent 1 X 1 pattern in LEED. Although the sample was heated up to 1200 DC to investigate the segregation of Ca impurities, NBISS measurements were made at room tem­perature without an electron shower to compensate the charging effect. Since the dose of the Heo beam during the measurements was kept below 108 atom/cm2, the decompo­sition of the specimen surface by sputtering could be ig­nored.

III. RESULTS Typical NBISS energy spectra for the MgO(OOI) surface

taken after annealing for a few minutes at various tempera­tures are shown in Fig. 2. The measurements were made with the use of an Eo = 1 ke V Heo beam under the fixed scattering geometry shown in the inset. The spectra are basically com­posed of two peaks, which can be assigned to Mg and Ca by comparison to the calculated binary collision energies indi­cated by arrows on the abscissa. The good agreement be­tween the calculated energy and the experimental peak posi­tion indicates that charging effects can be ignored in the present NBISS experiment. The spectral peak correspond­ing to Ca (hereafter we call "the Ca peak") is not detected at the surface annealed up to 800 DC and increases in intensity by raising the annealing temperature. One may think that the surface composition ofCa relative to Mg can be obtained from the ratio of the Ca peak to the Mg peak, but this as­sumption is not valid because of the fact that the intensity of the spectral peak is given as a function of very complicated factors such as scattering cross sections and neutralization/ ionization probabilities of He. Ionization of Heo effectively occurs in the close encounter with the target elements, pro­vided that the He Is orbital is sufficiently promoted in the molecular state with target atoms. 13 Hence, the ionization probability is strongly dependent on the identity of the target elements and the impact energy of the Heo. In fact, though ionization ofHeo on Mg2 + or Ca2 + occurs with a high prob­ability, ionization ofHeo on 0 2- is not explicitly observed at the present energy.

Although the determination of the surface composition is in general very difficult in ordinary ion scattering spectros­copy (ISS) and NBISS, it is known that the intensity vari­ation of the spectral peak for various scattering geometries can be used for the determination of the surface atomic ar­rangement. 14 Figure 3 shows the intensity of (a) Mg and (b) Ca peaks obtained at the Ca 2+ segregated MgO (00 I ) surface (-1200 DC annealing) as a function of a and the azimuthal angle 1/>; the detection angle of the scattered He +

was fixed perpendicular to the surface. In Fig. 3(a), the in­tensity decreases markedly for small a in the [100] and [110] azimuths, which is due to shadowing of the Mg2 +

ions by neighboring 0 2 - ions and Mg2 + (or Ca2 + ) ions,

respectively.14 A slightly more effective shadowing along the [100] azimuth compared to the [110] azimuth is due mainly to the small Mg-O distance compared with the Mg-

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3220

20'

01 ~

E ~

' .. :z:

'0 IS'

?: III C .. C

Souda et sl.: Segregation of Ca Ions at the MgO(001) surface

MgO(OO1) - Heo

Eo"lkeV

(0) Mg peak

o

~ ?: ',ji c .. C

1 20"

10' ~~ ________ -L ________ ~ ______ ~ __ ~ 5'

lal

o u

" .. :z:

(HO]

(t

1 20' h ••• r ..

10'

5'

[1001 [110] [010]

Azimuth(.-]

.' . •• i· '. • •. ~ '. .

~ .. N

1 20'

IS"

10' L--L ______ -L ______ ~ ________ ~ __ ~S'

(1-101 (100) (110] [010]

Ibl Azimuth (41)

FIG. 3. Intensity of He • ions scattered from (a) Mg' • and (b) Ca" ions at the Ca' t segregated MgO(OOl) surface measured as a function of the glancing angle a and the azimuthal angle q,; the detecting angle (J of scat­tered He' is fixed perpendicular to the surface.

J. Vac. Sci. Technol. A. Vol. 8. No.4. Jut/Aug 1990

3220

Mg distance. If a fraction of the 0 2 - ions were missing at the

topmost surface. shadowing in the [100] azimuth would be somewhat weakened. The intensity of the Ca peak shown in Fig. 3(b), on the other hand, is less dependent on the azi­muthal angle than that ofthe Mg peak; the dip due to shad­owing is relatively shallow and appears only for fairly graz­ing incidence. This suggests that the Ca2 + ions may be substituted for the Mg2 + ions, but are located at a rather different position compared with the Mg2 + ions.

Figure 4 shows the NBISS intensity of both Ca and Mg peaks as a function of a measured along the (a) [100] and (b) [110] azimuths; the scattering angle 0 L was fixed at 160·. As a decreases, the intensity drops to zero at a certain critical angle a c because of the shadowing effect; an increase in the intensity just before the shadowing effect begins is due to the "focusing effect" in which the Heo flux is concentrated just outside the shadow cone. 14 The focusing effect is more intense along the [110] azimuth compared to the [100] azi­muth for both Mg and Ca peaks. This is because the He +

1/1 C .. C

o

o

(a) (100) Azimuth

.. .. ..

..

Co peak ~ . . . . . .

'. "-Mg peak

. ......... .

. . , .. 10

. . I

20

T

(b) (110) Azimuth

o •

. ·0

. ' . . •

o. 0

" .

I I

30 40 IX. (deg)

Mg peak

I .. .. ... o 0 0 0

. . 10 20 30 40

« (deg)

so

....

so

. . . .

60

60

FIG. 4. Intensity of He' ions scattered from Mg' ~ (solid circles) and Ca' + (open circles) at the Ca" segregated MgO(OOl) surface as a func­tion of the glancing angle a of the incident Heo beam; the measurements were made at a fixed scattering angle 0,. = 160" in the (a) [1(0) or (b)

(110] azimuth.

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3221 Souda et al.: Segregation of Ca ions at the MgO(001) surface

ions formed during collision with target Ca2 + or Mg2 + ions are neutralized again effectively when they pass close to the neighboring 0 2 - ions on their outgoing trajectory. IS The shadowing of the Ca2 + ions occurs at a smaller a compared with that of the Mg2 + ions. If we assume that the Mg2 + and 0 2 - ions are located in the same plane at the Ca2 + segregat­ed surface, the small ac of the Ca peak relative to the Mg peak indicates that the Ca2 + ion is protruding from this plane.

IV. DISCUSSION

It is found in the preceding section that the Ca2 + ions

segregated to the (100) surface of MgO are substituted in part for Mg2 + ions. A quantitative structure analysis of this surface can be made from a further discussion of the polar angle scans of the NBISS intensity shown in Fig. 4. As de­scribed in the literature,14 the surface atomic structure can be determined if the shape of the shadow cone is known in advance. The shadow cone is easily calculated by using the interaction potential between the target atom and projectile ions. The interaction potential which is most generally used to describe ion scattering at low energies is a universal pair­wise potential based on a screened Coulomb potential:

V(r) = ZIZ2e2CP(r/a)/r, (1)

where ZI and Z2 are the atomic numbers of the projectile and target, respectively, and r is their separation. Various forms of the screening function cP and of the screening length have been proposed. Perhaps the most common form is the Moliere approximation to the Thomas-Fermi potential (TFM potential), 16 which takes the form

CP(x) = 0.35 exp( - 0.3x)

+ 0.55 exp( - l.2x) + 0.10 exp( - 6.0x). (2)

Various forms for the screening length have been proposed, and a common form is the value suggested by Firsov l7

:

a = CaF = C(0.8853 )aB (ZI 1/2 + Z2 1/2) - 2/3, (3)

where a B is the Bohr radius, 0.529 A. The adjustable param­eter C has been proposed by numerous researchers to im­prove agreement with the experimental results. 18-23 One method of obtaining a screening potential from low energy ion scattering is to compare the experimental shadowing critical angle a c with the calculations based on Eqs. (1)­(3). The method can be simplified by the assumption that the scattering is approximated by scattering from surface chains of atoms ("chain model"). This is the case for the [110] azimuth of the MgO(OOl) surface shown in Fig. 1, where the Mg2 + and 0 2 - chains alternate with each other. In the measured NBISS spectra, however, the intensity drop due to the shadowing effect is broadened by thermal vibra­tions, surface defects, and instrumental angular resolu­tion,23 and appears as in Fig. 4. According to the empirical rule based on a preliminary computer simulation, an experi­mental value of a corresponding to the 80% maximum in­tensity is approximately equal to the correct a c •

14 The a c

values for the Mg peak (Ca peak) intensities in Fig. 4, there­fore, are 18.0· (11.0·) and 17.0· (9.0·) in the [100] and [110] azimuths, respectively.

J. Yac. ScI. Technol. A, Yol. 8, No.4, Jul/Aug 1990

DI .. "

20

15

~ 10

5

o

o MgO(OOI)-He- Eo=lkeV

so

(110) Azimuth

• Experiment

- Calculation

ISO

3221

FIG. 5. Measured shadowing critical angles a c (solid circles) are compared with the theoretical critical angles calculated for a TFM potential with various values of the screening length parameter C.

For determination of the interaction potential, the mea­surements were made at the MgO(OOl) surface without Ca2 + segregation and the [110] azimuth was chosen be­cause the scattering may be chainlike. In fact, since the ener­gy spectra measured in the [110] azimuth exhibit no addi­tional peaks due to multiple scattering concerned with both Mg2 + and 0 2 - chains (zigzag scattering), the chain model is thought to be applicable to the present analysis. Solid cir­cles in Fig. 5 are the experimental values of a c as a function of the laboratory scattering angle 0 L between 30 and 160·; the calculated results on the basis of the chain model using the TFM potential with different C values are indicated by solid curves. Although the experimental points deviate sys­tematically from the calculated values, especially for small o L' the interaction potential seems to be well described with use of the TFM potential with a Cvalue ofO. 78. In the case of gas-phase scattering,24 it is known that a reasonable agree­ment to the TFM potential is obtained if C is chosen accord­ing to the formula

C=0.69+0.0051(ZI +Z2)' (4)

The C value of O. 78 for He on Mg is in excellent agreement with the value predicted by Eq. (4), 0.77. In many ICISS studies, moreover, the shadow cone is known to be repro­duced well by using the TFM potential with a C value of about 0.8. 19

-22 The experimental C value obtained here is

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3222 Souda etaL: Segregation of Ca Ions at the MgO(OO1) surface

thus found to be reasonable. It should be emphasized here that the possible deflection of the ion trajectories due to specimen charging, which is a common obstacle for ordinary ISS, can successfully be avoided in NBISS.

The position of the segregated Ca2 + ions at the MgO (00 1) surface can readily be determined, since we have already determined the interaction potential, or the shape of the shadow cone ofMg2 + ,for 1 keY HeO". The Mg2 + ions at the Ca2 + segregated surface are not displaced as much from the equilibrium position since a c for the Mg peak shown in Fig. 4 is almost the same as that shown in Fig. 5, although the angular dependence exhibits a tail at small a which is as­sumed to be scattering from surface defects. 25 The LEED pattern of the Ca2 + segregated surface, however, is as clear as the Ca2 + free surface and exhibits no lattice broadening or additional spots corresponding to surface reconstruc­tions. Hence, it is reasonable to assume that the concentra­tion of the Ca2 + ions at the surface is so small that the shad­owing of the Ca2 + ions measured in the [110] azimuth is mainly caused by the adjacent Mg2 + ions in the chain. With use of the shadow cone of He on Mg, the distance of the Ca2 + ions from the Mg2 + plane is determined to be about 0.4 ± 0.1 A,26 as schematically shown in Fig. 6(a). The pro­trusion of the Ca2 + ions at the MgO (00 1 ) surface is compa­tible with the fact that the ionic radius of the Ca2

+ ions is about 1.5 times as large as the Mg2 + ion.

With respect to the [ 100] azimuth, the intensity drops for

(Q) (110) Azimuth

(b) (1001 Azimuth

FIG. 6. (a) The location ofthesurfaceCa' + ions is determined using the a, value measured in the [110] azimuth [Fig. 4(b)] and the shadow cone of Mg' + calculated from the most probable interaction potential. (b) The Mg' + and 0'· ions are concluded to be located in the same plane within an experimental error of 0.1 A.

J. Yac. Sci. Technol. A, Yol. 8, No.4, Jull Aug 1990

3222

both Ca and Mg peaks are due to the shadow cone of the neighboring 0 2- ions as shown in Fig. 6(b). Although we could not obtain the interaction potential experimentally, a reasonable approximation for the interaction potential between He and 0 is to use the TFM potential and C = 0.8. The results are such that the Mg2 + ion is located in the same plane formed by the 0 2 - ions within an error margin of -0.1 A and that the Ca2 + ion is protruding by 0.4 ± 0.1 A from the surface. One interesting subject concerned with compound surfaces like MgO is the rumpling relaxation of neutral surfaces,'-9 i.e., the opposite displacement ofMg2 +

and 0 2 - ions. The structure of the rumpled MgO(OOl) sur­face has already been studied by LEED and RHEED, and the possibility of a small atomic displacement of - 0.1 A has been reported. The present NBISS study, however, is not capable of confirming this possibility because of the large experimental errors, which come mainly from the ambiguity of determining a c • Sample cooling or improvement of the instrument would be necessary for improving the accuracy of the experimental a c value.

V.SUMMARY

NBISS appears to be a promising technique to investigate surface structures of insulating crystals, as demonstrated for the MgO(OOl) surface. The interaction potential between Heo and Mg2 + is well described with use of the TFM poten­tial and a modified scaling factor C -0. 78 of Firsov's screen­ing length. The Ca2 + impurities included in the bulk are segregated to the (001) surface by annealing above 1000 ·C. The Ca2 + ions are substituted in part for the Mg2 + ions in the topmost surface and are protruding by 0.4 ± 0.1 A from the ordinary MgO(OOl) surface. The protrusion of the Ca2 + ions may be caused by the fact that ionic radius of Ca2+ is 1.5 times larger than that of Mg2+ , which implies that the segregation occurs so that the local strain due to substitution of Ca2 + for Mg2 + in the bulk is relieved.

I W. C. Johnson and J. M. Blakely, in Interfacial Segregation (American Society for Metals, OH, 1979).

'J. R. 8Iack and W. D. Kingery, J. Am. Ceram. Soc. 62,176 (1979). 3 A. Cimino, B. A. DeAngelis, G. Minelli, T. Persini, and P. Scarpini, J. Solid State Chern. 33, 403 (1980).

'Y. Fukuda and K. Tanabe, Bull. Chern. Soc. Jpn. 46,1616 (1973). 'J. W. Nelson and I. B. Cutler, J. Am. Ceram. Soc. 41, 406 (1958). 6R. C. McCune and P. Wynblatt, J. Am. Ceram. Soc. 66, III (1983). 7M. Prutton, J. A. Walker, M. R. Welton-Cook, R. C. Felton, and J. A.

Ramsey, Surf. Sci. 89, 95 (1979). "r. Gotoh, S. Murakami, K. Kinosita, and Y. Murata, J. Phys. Soc. Jpn.

SO, 2063 (1981). °T. Vrano, T. Kanaji, and M. Kaburagi, Surf. Sci. 134, 109 (1983). 10 H. Nakamatsu, A. Sudo, and S. Kawai, Surf. Sci. 194, 265 (1988). II R. Souda, M. Aono, C. O$hima, S. Otani, and Y. Ishizawa, Surf. Sci. 179,

199 (1987). 12 R. Souda, M. Aono, C. Oshima, S. Otani, and Y. Ishizawa, Surf. Sci. ISO,

L59 (1985): R. Souda and M. Aono, Nucl. Instrum. Meth. B 15,114 (1986).

13 R. Souda, T. Aizawa, C. Oshima, S. Otani, and Y.lshizawa, Phys. Rev. B 40,4119 (1989).

14M. Aono and R. Souda, Jpn. J. Appl. Phys. 24,1249 (1985). "R. Souda, M. Aono, C. Oshima, S. Otani, and Y. Ishizawa, Nucl. In-

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3223 Souda et al.: Segregation of Ca Ions at the MgO(001) surface

strum. Meth. B 15, 138 (1986). 16G. Moliere, Z. Naturforsch. Teil A 2,133 (1947). 17 0. Firsov, Sov. Phys. JETP 6,534 (1958). 18B. Poelsema, L. K. Verheij, and A. L. Boers, Surf. Sci. 64,554 (1977). 19 M. Aono, Y. Hou, R. Souda, C. Oshima, S. Otani, Y. Ishizawa, K. Mat-

suda, and R. Shimizu, lpn. I. Appl. Phys. 21, L67D (1982). 2°H. Niehus, Surf. Sci. 145,407 (1984). 21 S. H. Overbury and D. R. Huntley, Phys. Rev. B 32, 6278 (1985).

J. Vac. Sci. Technol. A, Vol. 8, No.4, Jul/Aug 1990

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22Th. Fauster and M. H. Metzner, Surf. Sci. 166, 29 (1986). 23 R. Souda, M. Aono, C. Oshima, S. Otani, and Y. Ishizawa, Surf. Sci. 128,

L236 (1983). 24D. I. O'Connor and R. I. MacDonald, Radiat. Elf. 34, 247 (1977). 25 M. Aono, Y. Hou, R. Souda, C. Oshima, S. Otani, and Y. Ishizawa, Phys.

Rev. Lett. SO, 1293 (1983). 26 The uncertainty of determining a" ± I·, is the main cause for the ambi­

guity of the location of Ca2', ± 0.1 A.

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